Patch "tcp: fix child sockets to use system default congestion control if not set" has been added to the 4.0-stable tree

This is a note to let you know that I've just added the patch titled

tcp: fix child sockets to use system default congestion control if not set

to the 4.0-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
tcp-fix-child-sockets-to-use-system-default-congestion-control-if-not-set.patch
and it can be found in the queue-4.0 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Neal Cardwell <[email protected]>
Date: Fri, 29 May 2015 13:47:07 -0400
Subject: tcp: fix child sockets to use system default congestion control if not set

From: Neal Cardwell <[email protected]>

[ Upstream commit 9f950415e4e28e7cfae2e416b43e862e8101d996 ]

Linux 3.17 and earlier are explicitly engineered so that if the app doesn't specifically request a CC module on a listener before the SYN arrives, then the child gets the system default CC when the connection is established. See tcp_init_congestion_control() in 3.17 or earlier, which says "if no choice made yet assign the current value set as default". The change ("net: tcp: assign tcp cong_ops when tcp sk is created") altered these semantics, so that children got their parent listener's congestion control even if the system default had changed after the listener was created.

This commit returns to those original semantics from 3.17 and earlier, since they are the original semantics from 2007 in 4d4d3d1e8 ("[TCP]:
Congestion control initialization."), and some Linux congestion control workflows depend on that.

In summary, if a listener socket specifically sets TCP_CONGESTION to "x", or the route locks the CC module to "x", then the child gets "x". Otherwise the child gets current system default from net.ipv4.tcp_congestion_control. That's the behavior in 3.17 and earlier, and this commit restores that.

---
 include/net/inet_connection_sock.h |    3 ++-
 net/ipv4/tcp_cong.c                |    5 ++++-
 net/ipv4/tcp_minisocks.c           |    5 ++++-
 3 files changed, 10 insertions(+), 3 deletions(-)

@@ -98,7 +98,8 @@ struct inet_connection_sock {
 	const struct tcp_congestion_ops *icsk_ca_ops;
 	const struct inet_connection_sock_af_ops *icsk_af_ops;
 	unsigned int		  (*icsk_sync_mss)(struct sock *sk, u32 pmtu);
-	__u8			  icsk_ca_state:7,
+	__u8			  icsk_ca_state:6,
+				  icsk_ca_setsockopt:1,
 				  icsk_ca_dst_locked:1;
 	__u8			  icsk_retransmits;
 	__u8			  icsk_pending;

@@ -187,6 +187,7 @@ static void tcp_reinit_congestion_contro
 
 	tcp_cleanup_congestion_control(sk);
 	icsk->icsk_ca_ops = ca;
+	icsk->icsk_ca_setsockopt = 1;
 
 	if (sk->sk_state != TCP_CLOSE && icsk->icsk_ca_ops->init)
 		icsk->icsk_ca_ops->init(sk);

@@ -335,8 +336,10 @@ int tcp_set_congestion_control(struct so
 	rcu_read_lock();
 	ca = __tcp_ca_find_autoload(name);
 	/* No change asking for existing value */
-	if (ca == icsk->icsk_ca_ops)
+	if (ca == icsk->icsk_ca_ops) {
+		icsk->icsk_ca_setsockopt = 1;
 		goto out;
+	}
 	if (!ca)
 		err = -ENOENT;
 	else if (!((ca->flags & TCP_CONG_NON_RESTRICTED) ||

@@ -437,7 +437,10 @@ void tcp_ca_openreq_child(struct sock *s
 		rcu_read_unlock();
 	}
 
-	if (!ca_got_dst && !try_module_get(icsk->icsk_ca_ops->owner))
+	/* If no valid choice made yet, assign current system default ca. */
+	if (!ca_got_dst &&
+	    (!icsk->icsk_ca_setsockopt ||
+	     !try_module_get(icsk->icsk_ca_ops->owner)))
 		tcp_assign_congestion_control(sk);
 
 	tcp_set_ca_state(sk, TCP_CA_Open);

Patches currently in stable-queue which might be from [email protected] are

queue-4.0/tcp-fix-child-sockets-to-use-system-default-congestion-control-if-not-set.patch